Marble is a naturally occurring form of calcium carbonate (calcite, CaCO_3) whic
ID: 963535 • Letter: M
Question
Marble is a naturally occurring form of calcium carbonate (calcite, CaCO_3) which used as a building material for exterior use. Calcite exhibits a K_sp of 4.5 times 10^-9, Carbonate ion, CO^-2_3, is a weak base in water (its conjugate acid is bicarbonate, HCO^-_3) having pK_e = 3.671. Would you expect to exhibit pH dependence in its water solubility? Why or why not? Buildings that are constructed of or covered with marble are regularly exposed to wetness from rain. What consequence does your expectation above imply for the durability of this material?Explanation / Answer
CaCO3 is a weak base in water.
The water solubility of CaCo3 is pH dependent. The reason is its dissociation in water produces Ca2+ and HCO3-. If acid will be added to it, then HCO3- will produce carbonic acid (H2CO3 => H2O +CO2). It is unstable and decompose to water and CO2. If gas will be released, then the equilibrium will shift towards formation of carbonic acid (Le Chatelier's principle). Hence, more and more CaCO3 will dissolve.
As there is a significant amount of sulfur dioxide and nitrogen oxides present in air, they form corresponding acid when comes in contact with water (rain water called acid rain). This low pH of water can damage the marble material slowly.